Not Enough Space on My C Drive

Open the disc manager to partition and format the former drive (start-run-diskmgmt.msc).

However, C and D must remain separate partitions - you cannot span them with a single boot partition.

You can run disc cleanup on C: (link below) and uninstall large programs from C: and reinstall them to

https://support.microsoft.com/en-us/help/4026616/windows-10-disk-cleanup

Free up space on the system volume

Use Disk Cleanup, or try to move files to an external location such as a CD, DVD, or external hard disk drive. Then, restart the installation. Make sure that required free space is available on the system drive.

To free up space on your hard disk, Disk Cleanup finds and then removes temporary files on your computer that you decide you no longer need. If you have more than one drive or partition on your computer, you are prompted to select the drive that you want Disk Cleanup to clean.

To start Disk Cleanup, click Start

, click All Programs, click Accessories, click System Tools, and then click Disk Cleanup.

@LTCBru  If the D partition was on a different physical disk, which is likely, then you cannot extend the C partition to span over to that physical disk.  Deleting a partition and extending an existing partition into that freed up space applies to cases where both partitions are on the same physical disk.  So if you want to use your 931 GB, you'll need to create a partition on that drive again, and then move some of your content from C over to D.  To figure out what's taking up so much space, download an app called TreeSize Free, run it as administrators so it can view all folders, and tell it to map out your C drive.  It will sort folders by largest to smallest, and when you drill into a folder, all of its subfolders will be sorted the same way, so it becomes very clear very quickly what's taking up space on your system.  If you want to move some of your entire profile folders, e.g. Desktop, Documents, Music, you can right-click those folders in their current location, then select the Location tab, and choose to move them over to somewhere on your D drive.  Or if you just want to move certain files, such as photos, music, videos, etc., then do that.  If it's application folders that are taking up space, you'll need to uninstall them and reinstall them, making sure to choose to install them onto a folder on your D drive during the reinstallation, rather than the default C drive location.